This invention relates to a process for the production of fermentable sugars such as glucose from wood or other biomasses.
It is known to pyrolyze pure cellulosE slowly at a low pressure at temperatures below 500.degree. C. to obtain a good yield of levoglucosan and other sugars or anhydrosugars. However, the use of wood for such pyrolysis results in poor yields of anhydrosugars such as levoglucosan. If hemicellulose is removed from the wood by acid treatment prior to the pyrolysis step, levoglucosan yields of up to 50% of the cellulose content can be obtained by slow heating under low pressure. However, at atmospheric pressure yields of levoglucosan are much lower, less than half of that obtained under vacuum. Further, smaller amounts of many other compounds are also present in the slow vacuum pyrolysis product. Among these are phenols from the lignin fraction of the wood, and furfural and its derivatives. Also, in the process of vacuum pyrolysis, the majority of the lignin content of the wood becomes converted to a solid char, suitable mainly for use as a fuel.
It is also known from U.S. Pat. No. 4,468,256 (Hinger) to produce glucose from vegetable raw materials by impregnating the raw material with dilute acid and subsequent heating to a temperature of above 250.degree. C. by means of high pressure steam. This heating step takes place in an extremely short time accompanied by a sudden pressure rise.
Generally, known methods of preparing fermentable sugar solutions from cellulosic material require digestion in acid. If this digestion is done using a strong acid, many undesirable and toxic side products are formed. If weak acid is used, poorer glucose yields and more dilute solutions are obtained. In this context, it is also known to use weak acid treatment of cellulosic raw material followed by enzymatic hydrolysis. This procedure can yield an improved product at higher concentrations compared to weak acid hydrolysis above. However, this process is relatively expensive.
There is a need for a process for producing fermentable sugar solutions from cellulose-containing biomass at a moderate cost and at relatively high yields so that the process may become competitive with other fuel production technologies, when the resulting sugars are a feedstock for alcohol production.